usb 3/thunderbolt in and out for connections to other systems, or a USB backup drive.

multiple midi ports, software configurable

patchable i/o - any analog jack can be assigned to input or output - 12 xlr/i/4 trs jacks configurable as balanced, hi-z, lo-z, line, mic, guitar/bass could be set up with several of the jacks configured as effects sends and returns to outboard gear. During looping, an audio loop could be assigned to that effects loop. Okay, it gets clumsy talking about this at times. There should be a headphone jack that can also be assigned output from all potential sources. Monitoring on the headphone jack you could set up all the i/o as effects sends/returns and record direct to the SSD

minimum: large dot matrix monochrome LCD main display for configuration. Status while running might have a big print and a fine print version for people who use it from a distance. There should be an easy to decipher layout of indicator lights both on the main unit, and on an accompanying control unit. More on that later. additional small displays for the i/o to label it dynamically as it changes

all parameters exposed through an open API. In addition to old fashioned MIDI as a last resort, it should be able to communicate and/or sync over wifi, usb, bluetooth, etherMIDI, etc. It should also be able to sync to a click track or CV clock - if possible it should be able to follow a human drummer or a tapping foot.

It should have 8 "tracks" that can contain additional sub tracks (takes/layers) that can be manipulated (on-off/reverse/time-pitch changes/effects) individually.

A master track consisting of midi data, audio data or both can be imported

all data can be exported at any time to USB storage or the local SSD

Multiple input means unit can also serve as 12 input recorder. If enough resources available additional inputs could be stacked up over USB. Some synths like the M-Audio Venom and the Casio PX-5s deliver a stereo mix over USB. Especially if they are class compliant, add USB audio to the point that the system is overloaded. You'll know it when you hear it.